1. Field of the Invention
The present invention relates to an LED (light-emitting diode) and more particularly to an LED structure provided with a wavelength converter. The wavelength converter can produce a light emission that is different from an original light emission in terms of the color of light emitted from an LED chip. For example, the wavelength converter can excite a yellow fluorescent material with a blue light emitted from the LED chip to yield an emission of white light that is synthesized from the blue and yellow light.
2. Description of the Related Art
In a conventional wavelength-converting LED 90 configured as shown in FIG. 3, an LED chip 91 that emits, for example, a blue light is mounted on one lead frame 92 and wired to the other lead frame 93 via a gold wire 94. The LED chip 91 is entirely sealed in an epoxy resin 95 that contains a fluorescent material 96 therein.
The fluorescent material 96 has a mean particle diameter D50 less than 5 μm as typically found in YAG:Ce, for example. Thus, only part of the blue light emitted from LED chip 91 transmits through the epoxy resin 95 and reaches the observer's eye, while the remainder of the light enters the fluorescent material 96 where it is converted into a yellow light, which reaches the observer's eye.
Accordingly, the observer receives the blue light and its complementary yellow light at the same time and recognizes them as white light. A thixotropic agent 97 may be added in the epoxy resin 95 by an appropriate amount (for example, 10% by weight) to prevent the fluorescent material 96 from precipitating before the epoxy resin 95 is cured due to a difference in specific weight. See Japanese Patent Publication No. 11-500584 (WO 98/12757), which is hereby incorporated by reference.
It has been discovered that the larger the particle diameter of the fluorescent material, the higher the wavelength conversion efficiency. This feature is shown in FIG. 2 with a curve D12 for a mean particle diameter D50 of about 12 μm; a curve D13 for a mean particle diameter D50 of about 13 μm; and a curve D24 for a mean particle diameter D50 of about 24 μm. Therefore, when the mean particle diameter D50 is set to be more than 5 μm, and preferably more than 10 μm, the wavelength-converting LED can be improved such that it has a much higher brightness.
In a practical pre-production prototype, however, the above-described precipitation of fluorescent material occurred in great quantities before the epoxy resin cured. Accordingly, even the conventional addition of the thixotropic agent to the epoxy resin is not sufficiently effective to prevent the precipitation, and allows most of the fluorescent material to precipitate. Thus, the blue and yellow lights can not be mixed sufficiently, resulting in variations of color and brightness in the light from the wavelength-converting LED. This is a problem because it is difficult to provide a wavelength-converting LED with a high brightness corresponding to an experimental value.